Device and method for the regeneration of mixed ion exchange resin beds

ABSTRACT

Equipment and procedures for regenerating ion exchange resin mixed beds, used for the desalination of water or of aqueous solutions from industrial processes (process solutions), that use two columns: the first treatment column (C 1 ), containing a mixed bed of ion exchange resins, in which the cation exchange resins are regenerated, and a second column (C 2 ) into which the anion exchange resins are transferred and regenerated, to be then reintroduced from the bottom into the above mentioned first column (C 1 ), where they rise through the cation exchange resins and intimately mix with the anionic exchange resins.

This application is a 35 U.S. C. §371 of PCT/EP97/00670 filed Feb. 13,1997.

This invention concerns a new device for regenerating mixed beds of ionexchangers used for the desalination of water or aqueous solutionsderiving from industrial processes (process solutions) and the method ofcarrying out this regeneration.

The technique of desalination by means of mixed beds of ion exchangerresins, described for the first time in 1951 (U.S. Pat. No. 2,578,937),is now widely used for the production of very low ion content water:among the most important applications are the production of steam-boilerfeed water and water for the electronic and pharmaceutical industries.

Another application of mixed bed processes concerns the desalination ofprocess solutions containing non-ionic organic compounds, as, forexample, molecules of pharmaceutical or foodstuffs interest. Whatcharacterizes mixed bed processes is the fact that water or the solutionto be desalinated is percolated through an intimate mixture of a cationexchanger and an anion exchanger.

Mixed bed treatment allows the reduction of the residual saline contentof treated water to quite lower levels as compared with desalinationthrough separate beds of two ion exchangers; in fact, whereas in thecase of separate beds, the fraction of ions removed is limited by theequilibrium value corresponding to the maximum degree of regeneration ofthe ion exchangers and hardly exceeds 99%, in the case of a mixed bedthere are no theoretical limits to the fraction of ions removed.

Furthermore, whereas during the treatment, for example, through separatebeds of cationic and anionic exchangers connected in series, the pH ofthe treated solution falls to very low levels in the column containingthe cationic resin, in the mixed bed pH remains near to neutral. Thischaracteristic allows, for example, the desalination of solutions of pHsensitive molecules.

Opposed to these advantages, the regeneration of mixed beds after use isunfortunately till now much more complex and costly than with separatebeds, because the cation exchangers and the anion exchangers must beseparated before regeneration, which is carried out with acids and basesrespectively and then they must be homogeneously remixed afterregeneration.

Separation and regeneration of exhausted mixed beds has been describedfor the first time in U.S. Pat. No. 2,771,424 (1956). A monography ofrecent processes was reported by B. Coulter, Ultrapure Water, November,1987.

In all regeneration processes, the resins are separated by hydraulicclassification by utilizing the different densities and granulometriesof the two exchangers.

Once separated, the resins can be regenerated in the same column whichcontained the mixed bed (internal regeneration) or one or both of themcan be transferred into one or more different columns where regenerationis carried out (external regeneration); they are then mixed in a specialmixer (or even in the column used for the regeneration of the cationexchanger) and then transferred into the column used for the mixed bed.Another possibility consists in transferring said resins, afterregeneration, into the column used for the mixed bed and mixing themtherein.

The external regeneration procedure requires a much more complex plant,and is therefore normally used only for the final desalination of waterto be fed to steam-boilers in thermoelectric or thermonuclear powerstations.

The most widely used process for small or medium sized units is theinternal regeneration. In this latter case the reagents for regenerationof the anion and the cation exchangers enter the column from the top andbottom respectively, either simultaneously or at different times(regeneration of the anionic exchanger is normally carried out first),while the exhausted regenerating solutions are collected from the samedischarge line provided with devices (strainers) able to retain theportion of resin situated near the interface between the resins.

This system is less costly but has two significant disadvantages:

1) the interface between the resins must be exactly maintained at thelevel of the discharge line, otherwise a part of the anionic exchangerwill be saturated by the acid used for the regeneration of the cationicexchanger or, vice versa, a part of the cationic exchanger will besaturated by the base used for the regeneration of the anionicexchanger.

This fact implies that it is not possible to handle a mixed bed withquantities of cationic resin different from those of the original designand that each variation of volume of the cationic resin, either due tothe normal swelling occuring during regeneration or to a possible lossof resin, will have negative effects on the following performance of thebed;

2) even if the level of the interface is maintained at the level atwhich the discharge line has been installed, there will always be acertain mixing between the two regenerating liquids or between oneregenerating liquid and the barrier water, in a substantial volumearound the interface between the resins.

Both problems 1 and 2 cause incomplete regeneration of that portionresins which is near to the interface; it means that a part of theanionic resin will be saturated with the cationic resin regenerationliquid and vice versa. This implies a lower exchange capacity in theregenerated resins at equal volumes and consumption of regeneratingliquid; furthermore, the portion of saturated anionic resin will releasesulphate or chloride ions and the portion of saturated cationic resinwill release sodium ions, so affecting the quality of water produced inthe successive desalination process (see e.g. G. J. Crits, Ion Exchange,Technology of Mixed Beds, Ultrapure Water, November, 1984).

These disadvantages can be reduced by introducing into the mixed bed,other than the two ion exchange resins, an inert separator consisting,for example of inert small beads with an intermediate density betweenthose of the two resins.

During hydraulic classification, the separator positions itself betweenthe two resins distancing them from the zone where mixing of the tworegeneration reagents takes place.

In this way, the partial saturation of the anionic resin is reduced aswell as the criticality of the level of the cationic resin, but thedesalination capacity per unit volume of bed will be inferior becausethe inert separator occupies a portion of the column volume.

On the contrary, external regeneration eliminates all of the problemsrelated to imperfect separation of the regenerating reagents in such wayassuring greater exchange efficiency, an improved degree of purity ofthe treated water and better repeatability of the desalination process:a recent mixed bed process with external regeneration is, for example,described in U.S. Pat. No. 4,472,282.

On the other hand, transferring the resins involves, as has already beenpointed out, complex equipments and troublesome handling of the same andlong overall regeneration times, thus making this method economicallyfeasible only for the treatment of big volumes of water, already with avery low salt content.

Consequently, the equipment complexity is justified only for big plantsand such operational work and long regeneration times are onlyacceptable when infrequent regeneration cycles (see e.g. B. L. Coulter,art. cit.) are required.

Another critical point in all mixed bed processes is the homogeneousmixing of the ion exchange resins once the regeneration is carried out.It is well known that the quality of water produced and the workingcapacity of the bed, largely depend on the quality of the mixing whichmust be as homogeneous as possible (see e.g. E. G. Baeva et al.,Development of a System for Mixing Ion Exchangers, Teploenergetika,1968).

In the devices known in the state of the art, mixing is always obtainedby fluidizing the bed by counter-washing with water and then blowing airfrom the bottom of the column. This method is also applied in externalregeneration units, with the sole difference that in the latter case,mixing is sometimes effected in a special apparatus instead of in thecolumn dedicated to the mixed bed.

Mixed beds obtained by this procedure can be, and usually are, lackingin homogeneity: in general, the upper part of the bed consists almostexclusively of the lighter anionic resin and the lower part almostexclusively of the cationic resin (data concerning lack of homogeneityin mixed beds are given by Baeva and S. Fisher, Trouble Shooting inMixed Bed Ion Exchange, Ultrapure Water, July-August, 1992). Only thecentral portion contains both resins mixed in quantities approaching theoptimal ratio; however, if, for example, transparent columns are used, asimple optical analysis shows that even in this portion, homogeneity isnot optimal: relatively large portions (of the order of 0.5 L in a 40 Lbed) in which cation exchangers prevail, alternate with zones of thesame size in which anion exchangers prevail.

In conclusion, available mixed bed techniques at the current state ofthe art, have disadvantages as compared to the conventional treatmentwith two or more separate beds, which are tied on the one hand togreater plant and operational complexity (above all if externalregeneration is used) and on the other hand to high handling costs and,as a consequence, are competitive for the production of ultrapure wateror of process solutions with very low ionic content, only in the case ofsolutions with already very low saline content, usually lower than thatfound in well water.

On the other hand, separate bed processes are not normally usable toproduce water or process solutions with a conductivity of less than 0.5μS/cm. Consequently, the production of water with very low conductivity(that is less than 0.5 μS/cm, preferably less than 0.25 μS/cm or even0.08 μS/cm, such as for example for thermonuclear plants) normallyrequires two treatment steps, in which only the second is carried outwith a mixed bed.

Furthermore, internal regeneration processes are normally used for smallmixed beds and as has been previously stated, they are ratherunsatisfactory, even for the quality of the deionized water produced ineach successive phase.

It is to the subject of this invention a new device for the regenerationand mixing of ion exchangers resins in a mixed bed and method foroperating it. This handling is much simpler in comparison to the unitswith external regeneration of the present state of the technology, itmaintains all of their advantages and even increases their performance,in particular thanks to the greater homogeneity of the mixed bedobtained by said method.

A further object of this invention is the method described below for thepreparation of a mixed bed of ion exchangers, characterized by greathomogeneity. This process does not require the use of air for mixing theresins and is applicable to units with external regeneration.

The simplicity and versatility of the plant object of this invention,make it usable even for small or medium sized applications and even inprocesses which require frequent regeneration, for the first timerendering possible, and economic, to obtain purified water with a puritydegree similar to those obtained by the best units with externalregeneration, even when starting from well-water or even from sea-water.

The mixed bed obtained by the method and the plant object of thisinvention, is furthermore able to reduce inorganic and organic ionicimpurities to extremely low levels in aqueous solutions of neutralorganic products (for example, molecules of pharmaceutical interest andtheir intermediates or sugar solutions or food products).

The scope and advantages of the device and the method according to theinvention are reached with the characteristics listed respectively inindependent claims 1 and 6. Advantageous applications of the inventionappear in the dependent claims.

Substantially, according to the invention, two columns are foreseen: afirst treatment column containing the mixed bed of ion exchange resins,in which the cation exchange resins are regenerated after thetreatment/desalination process, and then a second column into which theanion exchange resins are transferred and regenerated, to be thenreinserted at the bottom of the first column where they rise through thecation exchange resins present therein, intimately mixing themselveswith these, to give a reconstituted homogeneous mixed bed.

Further characteristics of the invention will be made clear by thedetailed description which follows, which refers to one of its purelyexemplary forms, therefore not limitative, and which is illustrated inthe annexed FIG. 1, in which:

FIG. 1 is a scheme of the device for the regeneration of fluidized bedsaccording to the invention.

Referring to this figure, the device object of this invention, consistssubstantially of two columns, one of which, indicated by C1 is dedicatedto contain the mixed bed and to the regeneration of the exhaustedcationic exchanger. The other, indicated by C2, is dedicated to theregeneration of the exhausted anionic exchanger, said columns beingjoined according to the diagram in FIG. 1. In the device of FIG. 1 inwhich the salient characteristics are shown, the open and close typevalves are indicated with the term V, the regulation valves with theterm VR and the circulation pump with the term P.

The functioning of the device of the invention substantially follows theoperations described below.

Initially, the anionic exchange resin is loaded into column C2 and thecationic exchange resin into column C1. Both of the exchange resins areregenerated separately and according to the supplier's instructions (forexample, for the regeneration of the anionic exchanger a strong base isused, usually sodium hydroxide at 4% w/w, whereas for the regenerationof the cationic exchanger, a strong acid is used, normally hydrochloricacid at 8 to 12% w/w or sulphuric acid at the same concentration).

For the cation exchanger, the acid solution is fed to C1 through valveV4, simultaneously discharging the exhausted regenerating solutionthrough valve V5 and regulating VR4 to maintain the liquid just abovethe level of the resin. Similarly, for the anion exchanger, the alkalinesolution is fed to C2 through V6, discharging the exhausted regeneratingsolution through V7 and regulating VR5 to maintain the liquid just abovethe level of the resin.

After regeneration, the ion exchangers are thoroughly washed withdeionized water through the same hydraulic circuits. Upon completion ofthe washing (which can be determined on the basis of the conductivity ofthe eluate or on the total volume of water used), both resins arecounter-washed with the flow recommended by the supplier for anexpansion of between 25% and 100% of the initial volume, supplyingdeionized water through valves V8 and V9 respectively and firstdischarging the air and then the water through valves V10 and V11respectively. At the end of this phase, the counter-washing is continuedin C2, whereas it is stopped in C1 by closing V8 and V10.

The cation exchange resin bed in C1 is again fluidized by opening valvesV12 and V13, starting the pump P and regulating the valve VR1 to obtainthe flow necessary to expand the bed of cation exchange resin up to200%-400% of its initial volume. Valves V9 and V11 are then closed andV1 is opened to pressurize the columns.

Valve V12 is closed then valve V14, which changes the flow of pumpedwater from C1 to C2, is opened simultaneously to V3, which causes theanionic resin to be transferred from C2 to C1.

In this way, the flow of water pumped through V14 pushes the fluidizedbed of the anion exchanger through V3 and into column C1 where theanionic resin, which is lighter than the cationic resin, rises throughthe fluidized bed of the cation exchanger mixing itself intimately withit.

When the anion exchanger has been transferred into C1, the pump P isstopped, valves V13, V14 and V3 are closed and the fluidized mixed bed,just obtained after the transfer of the anion exchanger from C2 to C1,is compacted by rapidly discharging water through valve V5. Valve V5 isclosed when the level of water in the column is just a little above thelevel of the resin.

The mixed bed obtained by this process results extraordinarilyhomogeneous as compared with those obtained by known methods (such as,for example, counter-washing with water and blowing air into the bottomof the column). The two resins are found to be homogeneously mixed forat least 80% of the column length (preferably for at least 90%); minimalnon-homogeneous residual zones are observed only at the top of thecolumn.

At this point, column C1 contains the regenerated mixed bed onto whichthe solution to be desalinated can be loaded (through V4); the resultingdesalinated water (ultra-pure water or desalinated solution of organiccompounds) is collected through valve V16. When the conductivity of thedesalinated solution is above the level of acceptability established forthe type of occuring purification, or when a predetermined volume ofsolution has been treated, the desalination phase is stopped and theseparation of the resins is begun for the regeneration of the same.

To this effect, column C1 is filled with water, through V8 for example,closing V1 and venting the air through V10; when the column is full, V10is closed and valves V1, V2, V15, and V12 are opened; the pump P isstarted and valve VR1 is gradually opened. In this way, the mixed bed inC1 is fluidized and the lighter anionic resin gradually separates out,above the cationic resin. When the flow, regulated by VR1 is increased,the upper part of the anionic fluid bed reaches the resin transfer lineand begins to pass over into column C2 through valve V2, together withthe flow of fluidization water. Valve VR1 is gradually opened to givethe flow necessary to expand the cation exchanger to a level just belowthe transfer opening of the anion exchanger (indicated by A in FIG. 1):valve V13 can be opened (partially or completely) to reach the maximumflow without excessive loss of load in the anion resin bed in C2.

When all of the anion exchanger is transferred into C2, the step isterminated by stopping the pump P and closing the valves which wereopened at the beginning of the transfer. The technicians can thenproceed with emptying excess water from the columns and with theregeneration of the ion exchangers by the procedure which has alreadybeen described.

In a variant of the invention, before the separation of the resins isstarted, a quantity of 4-12% w/w hydrochloric acid, greater than thatrequired for the regeneration of the cation exchanger, is fed into themixed bed in C1. In this case, after the separation, there will be noneed to regenerate the cation exchanger.

Apart from an increased acid consumption, this variant brings an easierand more complete separation of the ion exchangers and is oftenadvantageous in cases of desalination of concentrated solutions oforganic molecules.

The device and the method object of this invention, are very useful inthe pharmaceutical field, for example, because they consent to reducethe amount of inorganic and organic ionic impurities to extremely lowlevels in aqueous solutions of drugs and of diagnostics agents. Amongthese, it is worthwhile to mention compounds of non-ionic type, such as,for example, iodinated contrast agents for radiography or paramagneticcontrast agents for magnetic resonance imaging (MRI), products whichoften must be administered in particularly high concentrations and forwhich an high degree of purity is essential.

Among the neutral iodinated contrast agents for radiography, thefollowing can be, by way of an example, mentioned iopamidol, iomeprol,iohexol, ioversol, iopentol, iopromide, ioxilan, iotriside, iobitridol,iodixanol, iofratol, iotrolan, iodecimol, iopirol, iopiperidol.

Among the neutral paramagnetic contrast agents for MRI particularlypreferred resulted to be the gadolinium complex of10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1, 4,7-triaceticacid (gadoteridol).

The following examples are intended to illustrate the best experimentalconditions in which to apply the method, subject of the invention.

EXPERIMENTAL SECTION EXAMPLE 1

In a device according to the preceding description and to FIG. 1, thecolumns C1 and C2, which have effective volumes (measured from thesupport plate to the height of the feed distributor) of 36 and 22 L,were loaded respectively with 12 L of sulphonic cation exchanger Rohm &Haas Amberjet® 1200 Na and with 22 L of strong anion exchanger, type IRohm & Haas Amberjet® 4200 Cl. The ion exchangers were regeneratedrespectively with 26 kg HCl 10% w/w at a flow of 40 L/h and with 135 kgof sodium hydroxide 4% w/w at a flow of 90 kg/h. Both of the ionexchangers were then washed with deionized water until the eluateconductivity fell below 50 μS/cm. Then the anion exchanger wastransferred to C1 and mixed with the cation exchanger according to themethod subject of this invention. To the mixed bed so prepared, were fed100 L of a solution of 1.3 kg (22 mol) of NaCl in 100 L of water(conductivity 28,000 μS/cm) at a rate of 100 L/h.

The conductivity of the treated water was, at the beginning, a littleless than 1 μS/cm but rapidly fell, becoming stable, at 0.1 μS/cm until80 L of water had been treated. The conductivity then rose very rapidlyuntil it went out of the limit of the instrument (3,000 μS/cm)indicating that the bed was exhausted.

In order to have a measure of the total capacity up to saturation of theion exchangers, feeding of the saline solution was continued until theend and the eluate was collected in a second fraction. Then the mixedbed was washed with deionized water to a conductivity of 100 μS/cm,still collecting the eluate in fraction 2.

The total capacity of the anion and cation exchangers was determined bydividing the molar quantity of exchanged ions by the volume of the anionand cation exchangers. The molar quantity was obtained deducting fromthe total quantity of sodium chloride fed, respectively, the totalquantity of chloride ions determined with silver nitrate in fraction 2and the difference between the total quantity of chloride ions and thefree acidity titrated with caustic soda.

The total capacity of the anion exchanger resulted to be 0.84 mol/L(referred to the exchanger volume in the form of chlorine) and that ofthe cation exchanger 1.7 eq/L (referred to the exchanger volume in theform of Na), in accordance with what declared from the ion exchangermanufacturer.

The effective capacity of the mixed bed (defined as the quantity of ionsfixed before the conductivity rises above 0.5 μS/cm) resulted to be 17.6eq., which, when referred to the quantity of anion exchanger correspondsto 0.8 eq/L. This value is very high, in fact better than that foreseenon the basis of the data supplied by Rohm & Haas for a unit which usesseparate beds (Amberjet 4200 Cl co-flow engineering data).

EXAMPLE 2

For purposes of comparison, the same ion exchangers from Example 1 wereloaded into two separate columns, regenerated in the same conditions,counter-washed and washed with deionized water to the same finalconductivity.

The two columns were joined in series in such a way that the anionexchanger followed the cation exchanger.

A solution of sodium chloride in water in the same quantities as inExample 1 was fed to the two columns at a rate of 100 L/h.

The conductivity of the treated water was initially a little less than50 μS/cm, but rose moving up to 100 and then to 300 μS/cm up to 70 L oftreated water (pH alkaline). The conductivity then went out of the limitof the instrument (3,000 μS/cm) indicating that the deionizer wasexhausted.

As in the previous example, in order to have a measure of the totalcapacity up to saturation of the ion exchangers, feeding of the salinesolution was completed the eluate was collected in a second fraction.

Then the in series beds were washed with deionized water to aconductivity of 100 μS/cm still collecting the eluate in fraction 2.

Whereas the total capacity of the exchangers, determined by the samemethod as in Example 1, resulted practically identical to that ofExample 1, the effective bed capacity (defined in this case as thequantity of ions fixed before the conductivity rises above 400 μS/cm)resulted to be 0.68 eq/L.

This value is near to that expected, based on data supplied by Rohm &Haas for a separate bed unit like the one in this example (Amberjet 1200Na co-flow engineering and Amberjet 4200 Cl co-flow engineering).

The considerable advantages assured by the unit of Example 1 appearevident, both in terms of quality of the treated water and of capacitybefore exhaustion, at equal reagent consumption.

EXAMPLE 3 Desalination of a concentrated solution ofN,N′-bis(2,3-dihydroxypropyl)-5-[(hydroxyacetyl)methylamino]-2,4,6-triiodo-1,3-benzene-dicarboxamide

A) Solution ofN,N′-bis(2,3-dihydroxypropyl)-5[(hydroxyacetyl)methylamino]-2,4,6-triiodo-1,3-benzene-dicarboxamide.

90 kg ofN,N′-bis(2,3-dihydroxypropyl)-5-[(hydroxyacetyl)-methylamino]-2,4,6-triiodo-1,3-benzene-dicarboxamideobtained by the procedure described in EP 185130 were suspended in 400 Lof deionized water and heated under reflux. 310 g of 30% w/w sodiumhydroxide were added to the suspension. It was then heated to 120° C.under sealing conditions and this temperature was maintained for 1 hour.It was then cooled to 50° C. and 7.7 kg of 30% w/w sodium hydroxide wereadded gradually cooling to 40° C. over 2 hours. After a further 4 hoursat 40° C., it was cooled to 20° C. and pH was adjusted to 5.5 withhydrochloric acid. The solution obtained was loaded onto 160 L ofabsorbent resin R&H Amberlite 1600, feeding the eluate to ananofiltration unit equipped with a Desal DK4040 membrane. When theloading was completed, the resin was washed with 800 L of water at 40°C. collecting the eluate in the tank of the nanofiltration unit. Duringelution or at the end of it, the nanofiltration unit was started and theoperation continued until the volume of the solution in the unit wasreduced to about 200 L. At the same time, the elimination of the greaterpart of sodium chloride contained in the eluted solution was alsoobtained.

The obtained concentrated solution ofN,N′-bis(2,3-dihydroxypropyl)-5-[(hydroxyacetyl)methylamino)]-2,4,6-triiodo-1,3-benzene-dicarboxamide, which will from now on bereferred to as solution A, contains 80 kg of the desired product, about0.05 mol/L of organic ionic impurities (aromatic carboxylic acids) and0.03 mol/L of inorganic salts (prevalently NaCl)

B) Desalination of solution A.

200 kg of solution A at 40% w/w were fed at a rate of 40 L/h to the sameunit of Example 1, filled with the same quantities of the same ionexchangers, which were previously regenerated by the same method ofexample 1.

The eluate line was equipped with a conductivity analyser and also witha photometer to measure the absorbance at 280 nm, to detect the presenceof organic substances in the eluant.

The eluate was discarded until its absorbance began to rise rapidly,indicating the presence of the organic product in question.

From this moment, the eluate was collected in a tank up to exhaustion ofsolution A.

During the collection of this fraction which contains the greater partof the organic product, the conductivity remained below 0.1 μS/cm.

When solution A was finished, the mixed bed was washed with 30 L ofwater at the same flow rate and then again with 150 L of water at a flowrate of 100 L/h always collecting the eluate in the same tank of theproduct fraction.

Even during this phase, the conductivity of the eluate remained verylow.

The fraction corresponding with the desalinated product, which is freefrom chloride ions and carboxylic acids, was concentrated by evaporationto a viscous residue containing 15% of water. The product was thenisolated in a practically pure form (99%) by the addition of absolutealcohol at reflux temperature, followed by cooling and filtration.

EXAMPLE 4 Regeneration of the Mixed Bed after Treatment of the OrganicSolution of Example 3.

The mixed bed to be regenerated was counter-washed with 10 L of water.The water level was lowered to just above the level of the resin andthen, through the feed-line were fed, firstly 60 kg of 8.5% w/w HCl at aflow rate of 40 kg/h and then 200 kg of deionized water, the first 30 Lat the same flow rate and the remainder at a rate of 150 kg/h.

The cation and anion exchangers were then separated and the anionexchanger transferred to column 2, according to the description of thisinvention: the separation was found to be easy and clean-cut.

The anion exchanger was regenerated, washed and then retransferred tocolumn C1 where it was mixed with the cation exchanger, exactly asdescribed in Example 1.

EXAMPLE 5 Desalination test equal to that described in Example 3 on theregenerated mixed bed of Example 4

As a confirmation of the repeatability and reliability of the methodobject of this invention, the complete regeneration cycle according toExample 4 and desalination of a solution prepared according to point Bof Example 3, were repeated 42 times without significant functionalanomaly and with the same desalination efficiency.

EXAMPLE 6 Desalination of a solution of the non-ionic organic complex ofGadolinium of10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triaceticacid.

100 kg of an aqueous solution containing 25 kg of the compound inquestion, (solution A′), obtained by the method described in patentapplication EP 292689, was fed at a rate of 40 L/h to the same unit ofExample 1, filled with the same quantities of the same ion exchangers,previously regenerated according to the same method of Example 1.

The eluate line was equipped with a conductivity analyser and also witha photometer to measure the absorbance at 280 nm, to detect the presenceof organic substances in the eluate.

The eluate was discarded until its absorbance began to rise rapidly,indicating the presence of the organic substance in question.

From this moment the eluate was collected in a tank until the solutionwas finished.

During the collection of this fraction which contains the greater partof the organic product, the conductivity remained below 0.1 μS/cm.

When solution A′ was finished, the mixed bed was washed with 30 L ofwater at the same flow rate and then finally with 300 L of water at aflow rate of 100 L/h, always collecting the eluate in the same tank ofthe product fraction.

Even during this phase, the conductivity of the eluant remained very lowand the fraction corresponding to the desalinated product was shown tobe free from chloride ions.

What is claimed is:
 1. A device for the regeneration of mixed beds ofion exchange resins, including cation exchange resins and anion exchangeresins, contained in a first treatment column (C1) the devicecomprising: means for the separation of the anion exchange resins fromthe cation exchange resins in said first column (C1) by fluidizing themixed bed of resins which allows water to enter first column (C1) fromthe bottom, so that the lighter anion resins shift themselves on top ofthe cation resins; means for the transfer of the anion exchange resinsinto a second treatment column (C2) which column is in fluidcommunication with the first column (C1) by a conduit including atransfer opening (A) in the first column (C1), at a level above that ofthe mixed bed, a conduit connecting said opening (A) to the secondcolumn (C2), intercepted by a valve (V2), and further comprising meansfor expanding the cationic resin in the first column (C1) up to a leveljust below said opening (A); means for the regeneration of the cationexchange resins directly in the first column (C1); means for theregeneration of the anion exchange resins in the second column (C2)transferring the regenerated anionic resins from the second column (C2)to the first column (C1) and preparing a homogeneous reconstitution ofthe mixed resins and fluidizing the bed by pumping water into the secondcolumn (C2); said regeneration means consisting of an acid solution fedthrough a valve (V4), and the exhausted solution after regeneration isdischarged through a valve (V5) the second consisting of an alkalinesolution fed through a valve (V6), and the exhausted solution afterregeneration is discharged through a valve (V7); means for the transferof the regenerated anionic resins from the second column (C2) to thefirst column (C1) comprising a conduit intercepted by a valve (V3),which joins said columns in the region of their respective bases andwater pumped in the second column (C2) which transfers the bed ofanionic resins into the second column (C1) where it rises through thefluidized bed of the cationic resins mixing itself intimately with them.2. Method of regenerating mixed beds of ion exchange resins, comprisingcation exchange resins and anion exchange resins, contained in a firsttreatment column (C1) having a top and a bottom, said method comprisingthe following successive steps; (1) separating the anion exchange resinsfrom the cation exchange resins in the first column (C1) by introducingwater from the bottom of the first column (C1), to fluidize the mixedbed so that the lighter anionic resins rise above the cationic resins;(2) transferring the anionic resins from the first column (C1) to thesecond column having a top and bottom (C2) through a conduit connectingsaid first column (C1) and second column (C2), the conduit positioned ata higher level than that defined for the mixed bed and expanding thecationic resin bed in the first column (C1) to a level just below thatof the transfer line; (3) regenerating the cation exchange resindirectly in the first column (C1) by introducing an acid solution and indischarging the exhausted solution, maintaining the liquid level justabove that of the resins; (4) regenerating the anion exchange resin inthe second column (C2); (5) to the first column (C1) and preparing ahomogeneous reconstitution of the mixed resins and fluidizing the bed bypumping water into the second column (C2) to push the bed of anionicresins through a conduit with valves and through the bottom of the firstcolumn (C1) where the bed of anionic resins rises through the fluidizedbed of cationic resins mixing intimately with them.
 3. The methodaccording to claim 2, in which the cation exchange resins areregenerated prior to separating the anion exchange resins from thecation exchange resins.
 4. The method according to claim 2, wherein theregeneration of the cation exchange resins takes place by introducinginto the first column (C1) of an acid solution in quantities greaterthan those necessary for the regeneration of said cationic resins. 5.The method according to claim 4 wherein the acid solution contains 8% to12% w/w hydrochloric acid or sulfuric acid.
 6. The method according toclaim 2, wherein the anionic resins are regenerated in column 2 (C2) byintroducing an alkaline solution and discharging exhausted solutionwhile maintaining the liquid level just above the level of the resins.7. The method according to claim 4, wherein the alkaline solutioncontains 4% w/w sodium hydroxide.
 8. The method according to claim 2,wherein after regeneration, the resins are washed with deionized waterand counter-washed with deionized water thereby reaching an expansion of25% to 100% of their initial volume.